Blood vessels are formed by two processes. Vasculogenesis is the creation of new vessels from differentiating mesoderm, and angiogenesis is the extension and remodeling of existing vessels. Although angiogenesis is an essential component of normal physiological processes and responses to injury, pathological disturbances of angiogenesis underlie tumor development, hereditary vascular disorders and other diseases. The molecular basis of angiogenesis remains incompletely understood despite recent advances. Although numerous receptor and ligand families are essential in mediating angiogenesis, few intracellular signaling intermediates have been demonstrated to be specific or essential for this process. Cytokines of the TGF- beta superfamily, which signal exclusively through a unique set of receptor serine-threonine kinases, play important roles in many developmental processes, including angiogenesis. TGF-beta family ligands transmit their signals directly from cell surface receptors to the nucleus by a highly conserved family of transcription factors called Smads. Published data and our preliminary experiments demonstrate that two of these proteins, Smad1 and Smad5 are essential for angiogenesis in the developing murine embryo. Smad1 and Smad5 are key mediators of angiogenesis, and are among the first signaling intermediates to be identified as essential for this process. Our long range goal is identification of upstream activators and downstream effectors of Smad1- and Smad5-mediated angiogenesis. Our hypothesis is that during embryonic angiogenesis, Smad1 and Smad5 mediate a TGF-beta receptor-activated signaling pathway, which, through interaction with specific DNA binding proteins, activates or represses target genes essential for angiogenesis in the developing mouse embryo. We will use a combination of genetic, cell biological and biochemical techniques to define the signaling pathways that are mediated by Smad1 and Smad5 during embryonic angiogenesis.